Vanadium is a highly versatile metal and finds use principally as an alloying element in iron where the addition of amounts as small as 0.1 percent or less to carbon steel can greatly enhance strength, toughness and ductility. These high-strength low alloy (HSLA) steels are used widely in the construction of bridges, buildings, pipelines and the like. This versatile metal is also useful in the preparation of high performance metals, such as those comprised of titanium, aluminum and vanadium. Additionally, vanadium is employed in catalysts used in the manufacture of sulfuric acid and rubbers.
However, even though vanadium is considered to be a fairly common element, it is rarely found in sufficient concentrations in the earth's surface where it can be mined economically for the element alone.
Vanadium produced outside of the United States is usually obtained as a by-product in the processing of iron ores, such as titaniferous magnitite. The major reserves of such ores are found in such countries as Russia, the People's Republic of China and the Republic of South Africa.
Until recently, more than one half of the vanadium mined in the United States was obtained as a by-product in the mining of uranium. However, due to the reduction in nuclear power plant construction, the current demand for uranium has diminished and, hence, the United States vanadium industry has also suffered. Thus, due to the downturn in the development of new uranium mines in the United States and elsewhere, vanadium suppliers may be unable to satisfy the demand due to changing world conditions. Although the Republic of South Africa is one of the world's largest suppliers of vanadium, its present political climate does not insure a continued source of this valuable metal. Russian vanadium is not sold on the world market and the Chinese supply has been erratic.
More recently, petroleum and petroleum by-products have been investigated as a possible source of vanadium since nearly all crude oils contain at least trace amounts of vanadium. The presence of vanadium is, of course, undesirable in the processing of crude oil since it contaminates the hydrocracking and hydroforming catalysts. Power plants that burn crude oil having a high vanadium content are known to end up with vanadium-bearing fly ash and vanadium-bearing boiler residues.
In the mid 1970's a process was developed by the Exxon Research and Engineering Company for increasing the quantity of distillate from ultra-heavy crude oils. This method, referred to as "Flexicoking" produces (a) demetallized coker gas oils, (b) naphtha, (c) coke gas and (d) product coke. Flexicoking is a combination of fluid bed coking and coke gasification which concentrates 99 percent of the contained metals into a coke purge stream product, containing high levels of vanadium and nickel.
In practice, Flexicoking is a coking-gasification process which is carried out in three vessels (a fluid coker, a heater and a coke gasifier) containing fluidized coke solids. The three vessels are interconnected to permit transfer of solid from one vessel to another. Vacuum distillation bottoms are fed to the Flexicoker and about 95 percent of the coke that would normally be produced in a fluid coker is consumed in the production of a low BTU gas. The metallics are concentrated in the remaining coke which is appropriately referred to as "Flexicoke". Thus, Flexicoking at high coke gasification levels can result in a high concentration of metals in the Flexicoke product. This high metals content coke is a potential source of valuable vanadium and nickel.
The Flexicoke product can be a composite of three streams from the Flexicoke processing unit. A bleed coke stream from the heater bed to control the level of metallic buildup, and two streams from the product gas, i.e., tertiary cyclone fines and venturi scrubber cake. For example, Flexicoke from the heater bed will usually contain from about 1 to about 5 weight percent vanadium calculated as vanadium pentoxide, whereas Flexicoke from the wet scrubber may contain as much as 20 weight percent vanadium calculated as vanadium pentoxide. The cyclone venturi fines will normally contain 8 to 12 weight percent vanadium pentoxide. Thus, although the Flexicoke employed in the examples which follow is a blend of residues from each of the three sources, one may find it more desirable to use Flexicoke from just a single source, such as the wet scrubber cake which will usually contain the highest concentration of vanadium.
In practice, the vanadium recovered in the United States from utility ashes and refinery residues is mainly of foreign origin. Domestic petroleum crudes rarely contain more than 15 ppm vanadium and usually much less. In contrast, Venezuelan crudes average about 200 ppm vanadium and sometimes as high as 350 ppm. Hence, utility companies that burn Venezuelan crudes or Flexicoke units that employ Venezuelan crude present an attractive source for this desirable metal. In addition to vanadium, Flexicoke contains recoverable amounts of nickel. However, to date no satisfactory process has been found by which good recovery of vanadium is achieved with good separation from nickel and other impurities without employing expensive reagents and highly specialized equipment.
McCorriston in U.S. Pat. Nos. 4,389,378; 4,472,360 and 4,536,374 discloses processes for recovering vanadium from cokes and ashes derived from heavy oils. In these processes, the coke or ash is admixed with an alkali metal carbonate and then heated to an elevated temperature below the fusion point of the reagent. The mixture can then be subjected to leaching for the recovery of vanadium. Nickel, if sought, however, must still be recovered.
Barclay in U.S. Pat. No. 4,420,464 discloses, among other things, the recovery of vanadium from Flexicoke in which the Flexicoke is gasified to produce medium BTU heating fuels and withdrawing from the gasifier, a molten salt pool which pool is then treated with an oxygen-containing gas to convert the vanadium into a water-soluble form. After quenching in an aqueous solution and ash removal, the solution is reacted with carbon dioxide to produce alkali metal bicarbonates and hydrogen sulfide. The bicarbonates precipitate and vanadium can be recovered from the solution. Nickel, if sought, must be recovered by a separate mechanism.
Metrailer, et al., in U.S. Pat. No. 4,203,759 also disclose recovery of vanadium from partially gasified coke. The disclosed process involves heating the partially gasified coke in an oxidizing atmosphere at a temperature below the fusion point. The vanadium can then be leached with sulfuric acid, and, depending upon the particle size, up to 99 percent of the vanadium can be recovered. Nickel must be recovered by a separate mechanism.